This paper reports on the development of an expert system for the design of the physical fixture for the fixturing of prismatic parts on machining centers. A brief description of the Brite project 3480 IDEFIX in which the system was developed is given. With as input the faces on which positioning, clamping and/or supporting should be done the expert system generates the needed assembly of modular fixturing elements (i.e. the physical fixture). New in the approach is the description of modular elements by means of form features. This method permits the description of an arbitrary modular fixturing system (with the only restriction that it has to be based on a grid of holes) and the use of the same logic to determine the assembly of modular elements.